1. Field of the Invention
The invention relates to the field of aligning a laser beam with circuit elements in an integrated circuit.
2. Prior Art
In the fabrication of integrated circuits, particularly metal-oxide-semiconductor (MOS) memories, redundant elements such as redundant rows and columns are sometimes fabricated on each circuit or die. If it is determined that the memory has a faulty element, a discretionary connection (or disconnection) is made to effectively decouple the faulty element from the memory and couple the redundant element into the memory.
Most often, these redundant elements are selected by electrically blowing fuses such as polysilicon fuses, or aluminum fuses. (Such fuses are also used to program read-only memories.) See U.S. Pat. No. 3,792,319 for a description of a polysilicon fusible link.
An alternate method discussed in the prior art is blowing these fusible links by laser irradiation. (See U.S. Pat. No. 4,228,528). To blow these links with a laser beam, it is necessary to precisely align the laser beam to the fusible link. While this may be done manually, an automated system of alignment is necessary for high volume production.
A prior art method for aligning a laser beam to elements in an integrated circuit is described in conjunction with FIG. 1.